1. Field of the Invention
This invention relates to an electromagnet having a magnetic shield for use in a nuclear magnetic resonance diagnostic apparatus or the like and, more particularly, to a structure whereby a highly uniform magnetic field can be generated.
2 Description of the Related Art
FIG. 4 is a cross-sectional view of a conventional magnetically shielded electromagnet of this kind taken on a vertical plane containing the center axis thereof. For example, this electromagnet is constructed as a superconducting electromagnet. A coil 1 which generates a uniform-magnetic-field is formed by winding a superconducting wire so as to have a hollow cylindrical shape and to be symmetrical with respect to a center point 0 on the center axis CL. A cryostat 2 contains the uniform-magnetic-field coil 1 and a liquid helium 3 for cooling the uniform-magnetic-field coil 1 at an extremely low temperature to maintain this coil in a superconducting state. A magnetic shield 4 is arranged to surround the cryostat 2. The magnetic shield 4 has a hollow cylindrical yoke 5 formed of a magnetic substance, e.g., iron and having a center axis and a center point coinciding with the center axis CL and the center point 0, and a pair of end plates 61 and 62 also formed of a magnetic substance, e.g., iron. The end plates 61 and 62 are ring-like circular plates having generally central holes 61a and 62a respectively which have a diameter approximately equal to that of an inner cylindrical space 7 which is formed at the center of the magnet. A hole 8 is also formed in the end plate 61. A service port 2a extends through the hole 8. The service port 2a projects from one end of the cryostat 2 to outside of the magnetic shield 4 in order to guide a lead 1a for introducing a current to the coil 1 and to serve a piping for supplying the liquid helium 3, and the like.
In the thus-constructed electromagnet with the magnetic shield, when the uniform-magnetic-field coil 1 is energized in a superconducting condition, a magnetic flux is formed in the inner cylindrical space 7 in the direction of the Z-axis corresponding to the center axis CL. The greatest part of this magnetic flux circulates in a loop by flowing into the end plate 61 having a large relative magnetic permeability, flowing through the cylindrical yoke 5 in the direction opposite to that of the flow inside the inner cylindrical space 7 and returning into the inner cylindrical space 7 through the end plate 62. The magnetic shield 4 thereby limits the magnetic field leakage to the outside. A uniform magnetic field necessary for nuclear magnetic resonance diagnosis can be obtained in a spherical region 9 indicated by the broken line around the center point o in the inner cylindrical space 7.
The shape of the magnetic shield 4 and so on, as well as the coil 1, influence the magnetic field generated in the spherical region 9. It is therefore necessary for the magnetic shield 4 to have a strictly symmetrical structure.
In the conventional magnetically shielded electromagnet, specifically, in the construction of the magnetic shield 4, the magnetic resistance is locally increased at the hole 8 owing to the existance of this hole 8 formed in the end plate 61, and non-uniform magnetic field gradient components is generated in the spherical region 9 because of this asymmetric construction.
These conditions will be described below in more detail. Assuming rectangular coordinate axes X, Y and Z as shown in FIG. 4, and assuming that the hole 8 formed in the end plate 61 is located on, for example, the Y-axis, components relating to the Z- and Y-axes, i.e., magnetic field gradient components represented by functions of Z, Y, ZY, Z.sup.2 Y and the like occur. If the hole 8 is located on the X-axis, components relating to the Z- and X-axes, i.e., magnetic field gradient components represented by functions of Z, X, ZX, Z.sup.2 X and the like appear. If the hole 8 is located at a position on the X-Y plane excepting on each axis, magnetic field gradient components represented by a function of Z, X, Y, ZX, ZY, XY, X.sup.2 --Y.sup.2, Z.sup.2 X, Z.sup.2 Y, ZXY and the like are generated.
The conventional electromagnet therefore requires a magnetic field compensation means for removing the non-uniform magnetic components caused by the hole 8. More specifically, it is necessary to add various types of magnetic field compensation coils and to increase the output to enable magnetic field compensation.
A problem relating to magnetic attraction between the uniform-magnetic-field coil 1 and the magnetic shield 4 is also encountered. That is, the magnetic attraction force acting from the center point 0 toward the hole 8 is smaller than the magnetic attraction force acting in the opposite direction, and the uniform-magnetic-field coil 1 is attracted in the direction from the hole 8 to the center point 0. It is therefore necessary to reinforce the support for maintaining the uniform-magnetic-field coil 1 in the normal position A complicated and expensive support structure is therefore required.